The invention relates to a surface acoustic wave (xe2x80x9cSAWxe2x80x9d) filter. The selectivity of a SAW filter is greatly impaired by electromagnetic crosstalk. Crosstalk is significantly caused by closely adjacent filter structures, particularly closely adjoining structures of interdigital input and output transducers and their power connection tracks, also called pads.
A conventional transducer and pad topology is shown diagrammatically in FIG. 1. Input and output transducers 1 and 2, for example of a Z-path filter, are shown with diagrammatically illustrated reflectors 3, 4. Electromagnetic crosstalk is caused by two mechanisms, namely, by capacitive crosstalk between closely adjacent pads 5 and 6xe2x80x94symbolized by an equivalent circuit capacitance xe2x80x9cCxe2x80x9d between the pads indicated by dashed lines, and, by inductive crosstalkxe2x80x94symbolized in FIG. 1 by magnetic field lines H that enclose adjacent current flow lines partly identified by an arrow.
Typically, both mechanisms, i.e., inductive and capacitive crosstalk, occur simultaneously. Their respective shares of electromagnetic crosstalk depend on the filter parameters, with inductive crosstalk becoming increasingly significant at high frequencies and low impedances.
Typical SAW filters, the selectivity of which is considerably impaired by electromagnetic crosstalk, are acoustically transversely coupled filters, e.g., symmetrically operated transverse-mode-coupled resonator filters (also called transversely coupled resonator (xe2x80x9cTCRxe2x80x9d) filters) of the type described in xe2x80x9c1996 IEEE Ultrasonics Symposiumxe2x80x9447-51xe2x80x9d. According to FIG. 2 on page 48 of the Symposium document, the input or output transducers are split along their longitudinal direction, i.e., in the direction of the acoustic track, and the part or partial transducers obtained are electrically serially interconnected. As a result, the capacitive crosstalk between the split partial transducers and the transducer element directly adjoining in the adjacent acoustic track can be compensated. Such compensation requires symmetric operation by at least one filter port, i.e., at least at the input or output transducer. In such case, the crosstalk cancels due to the different polarity of the voltages applied and the equal amount of the equivalent capacitance. Disadvantageously, the impedance of the corresponding connections is increased to twice the initial value.
Other conventional filters that are impaired in their selectivity by electromagnetic crosstalk includes Z-path filters, which have become known from, for example, German Published, Non-Prosecuted Patent Application DE A-40 18 784. Such filters couple input and output transducers disposed on separate acoustic tracks to one another through correspondingly inclined reflectors. The document proposes an interconnection of two part or partial filters in the form of a half bridge in which electrical signals cancel and acoustic signals are constructively superimposed by a suitable additional phase shift. A significant disadvantage of these filters is that they require twice the space and have an extremely elaborate and complicated pad structure.
In the case of in-line filters, i.e., filters having input and output transducers disposed on a common acoustic track, the electromagnetic crosstalk is kept low by relatively large transducer spacing and the minimization of xe2x80x9chotxe2x80x9d, i.e., signal-conducting transducer, areas.
It is accordingly an object of the invention to provide a surface acoustic wave filter that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that has at least two electromagnetically coupled transducers for compensating filter capacitive crosstalk and that has a greatly improved selectivity in comparison with conventional filters without increasing the impedance of the filter.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a surface acoustic wave filter that can be operated symmetrically, including electromagnetically coupled IDT transducers split into partial transducers, the IDT transducers having electrical connection pads and ports, the partial transducers connected in parallel, the partial transducers and the electrical connection pads disposed to make, in the case of symmetric operation, crosstalk capacitances between adjacent ones of the pads and the IDT transducers approximately equal in amount at a corresponding one of the ports and at voltages of opposite polarity for balancing the crosstalk capacitances.
The invention provides a SAW filter that can be operated symmetrically. The filter exhibits electromagnetically coupled IDT transducers on mutually adjacent acoustic tracks and the IDT transducers are split into partial transducers. The partial transducers are connected in parallel. The partial transducers and electrical connection pads are disposed such that, for symmetric operation, the crosstalk capacitances between adjacent pads and IDT transducers are present at the corresponding port at voltages of the same amount but are provided with different signs. Thus, they are balanced.
In an advantageous development of the invention, the interdigital transducers are split into partial transducers. The routing of their pads and power connection tracks are modified such that, in addition, the flux directions of the magnetic couplings between the partial transducers and the transducers of the adjacent acoustic track are oppositely directed so that the total induced magnetic flux is zero. As such, the two main causes of the crosstalk of the SAW filter are eliminated and the transfer characteristic is improved.
In accordance with another feature of the invention, the IDT transducers have input and output transducers and the input and output transducers are split into the partial transducers.
In accordance with a further feature of the invention, the input and output transducers are split into the partial transducers longitudinally to acoustic tracks of the filter.
In accordance with an added feature of the invention, the input and output transducers are split into the partial transducers parallel to acoustic tracks of the filter.
In accordance with an additional feature of the invention, the partial transducers of the input and output transducers are connected in parallel.
In accordance with yet another feature of the invention, the partial transducers of the input and output transducers are connected differently.
In accordance with yet a further feature of the invention, at least one of the partial transducers is connected in parallel and at least another one of the partial transducers is connected in series.
The polarity of the electric drive is transposed in one of the partial transducers by a modified routing of the pads. The desired acoustic operation of the filter is maintained in the configuration by mirroring the relevant partial transducer in parallel with the preferred direction of acoustic propagation, i.e., by mirroring at its longitudinal axis.
In accordance with yet an added feature of the invention, one of two of the partial transducers is mirrored parallel to a direction of acoustic propagation.
The transposition of the polarity of the electrical drive can also be achieved if a partial transducer is displaced by an odd-numbered multiple of the half wavelength of the surface acoustic wave with respect to the other partial transducer.
In accordance with yet an additional feature of the invention, one of the partial transducers of one of the IDT transducers is displaced with respect to another one of the partial transducers of the one of the IDT transducers by an odd-numbered multiple of a half wavelength of a surface acoustic wave.
In accordance with again another feature of the invention, the IDT transducers have input and output transducers and the input and output transducers are split into the partial transducers at least one of transversely and in a direction transverse to an acoustic track.
In accordance with again a further feature of the invention, the partial transducers are connected in parallel.
In accordance with a concomitant feature of the invention, a pad of the pads routing in one of two of the partial transducers is selected to transpose a polarity of an electrical drive.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a surface acoustic wave filter, it is, nevertheless, not intended to be limited to the details shown since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.